1. Field of the Invention
This invention relates to a dial indication type measuring instrument, and more particularly to a mechanism for correcting a shift of a needle.
2. Description of the Prior Art
There have heretofore been known dial indication type measuring instruments such as dial indication type slide calipers and a dial indication type depth gauge, in both of which, a needle in a dial indicating portion is driven by a rack and a pinion, whereby a measured value is indicated by the needle. In the dial indication type measuring instrument described above, a measured value is easily readable and the high precision measurement is facilitated as compared with the construction of slide calipers in which a measured value must be read from both graduations of a main scale and an auxiliary scale for example, whereby the dial indication type measuring instrument has been used widely.
Now, to meet the requirements for higher accuracy in measured values and for compactness in size of the measuring instrument as a whole, the measuring instrument of the type described should have a rack and a pinion, which are very delicately finished. For example, in the dial indication type slide calipers having the minimum graduation of 0.01 mm of a dial plate, the module is determined to be 0.1-0.2 thereof, and moreover, the rack and the pinion are in mesh without a play. Because of this, if any foreign matter or the like intrudes a space formed between the rack and the pinion and adheres thereto, then, when the pinion is getting over the foreign matter or the like, a shift in position tends to occur in the meshing engagement between the rack and the pinion, thus resulting in a shifted zero position of a needle in a dial indicating portion.
In the above-described case, heretofore, such a correcting method has been taken that a thin sheet is inserted between the rack and the pinion, whereby the slider is caused to move in a state where the pinion is floating up from the rack and so forth, so that the shift in position of the needle can be corrected. However, the following disadvantage has been presented by this correcting method using the thin sheet. More specifically, even if the forward end of the thin sheet is pushed into the space formed between the rack and the pinion, the thin sheet cannot be snugly inserted into the space in many cases. If the thin sheet is forcedly pushed into the space, then the pinion or the like tends to be damaged. Furthermore, it is not desirable that the direct contact of the thin sheet with the rack and the pinion, which have been finished with high accuracy. Moreover, when the thin sheet is inserted into or withdrawn from, the pinion tends to be rotated, it has been difficult that the position of meshing engagement between the rack and the pinion is corrected in a manner as desired. Further, since it has been very difficult to move the slider for a large distance while the thin sheet is inserted into the space between the rack and the pinion, the slider can hardly be moved after the insertion of the thin sheet. In consequence, the slider has been stopped by visual observation at an intermediate position where an end place such for example as 2.00 mm or 5.00 mm of the main scale graduation becomes zero, not at the true zero position where a main scale jaw is brought into contact with a slider jaw, and at this time, the needle in the dial indicating a portion is set at zero and so on. Therefore, in view of this fact, it has been difficult to achieve a correct zero adjustment of the needle, and, in some cases, the dial plate in the dial indicating portion should be rotatably adjusted even after the shift in position of the needle has been corrected. Furthermore, it has been normal to provide the rack in a groove formed in the main scale, and, in order to insert the thin sheet into the space between the rack and the pinion, the groove should be made large and long beyond necessity, wereby the labor for finishing is increased, thus presenting a possibility of decreasing the mechanical strength of the main scale.